There are three basic types of spindle drives employed in most present day disk drive systems. Some drives have an AC synchronous motor coupled to the spindle by means of a belt and pulleys. This utilization of AC power to drive the motor makes the motor speed dependent on the source frequency. Consequently, if it is desired to operate the drive on various frequency sources, different pulley ratios must be employed to maintain a given speed. Also, these pulley type drives tend to be relatively complex and expensive. Also they are relatively inefficient and are generally characterized by excessive power consumption. Still further, in order to be economically competitive, that type drive utilizes relatively low cost AC motors and hence they are dependent upon the belt drive system inertia to smooth the torque ripple inherent in such motors since the motors have a limited number of magnetic poles.
In the second type of system, the spindle is driven directly by a conventional DC motor whose speed is controlled by the output of a tachometer connected with the motor in a servo loop. While that arrangement avoids the requirement of belts and pulleys, it has other problems. More particularly, the commmutation or brush noise inherent in such DC motors can prove detrimental to the high performance operation of the overall data recovery system in which the drive is a part. Also that type drive requires frequent maintenance in the form of motor brush replacement. Also, while those direct drive systems avoid the cost of belts and pulleys, they still require a tachometer which is a relatively expensive item and therefore makes the overall system fairly expensive.
The third type spindle drive used in systems of this type employs a servo driven DC motor linked to the spindle by way of a pulley drive. Therefore, it has disadvantages of the above two described systems.